The utilization of lignocellulosic waste materials, such as cornstalks, sawdusts, straws, bagasse, and the like, has been the subject of strong interest recently, particularly with respect to utilization of such agricultural and waste materials for developing alternate sources of fuels, chemicals, glucose and the like. Lignocellulose is commonly referred to as biomass.
Lignocellulosic materials include three principal components—cellulose (30-40%), hemicellulose (20-30%), and lignin (5-30%). In its natural state, cellulose is highly crystalline in structure with individual cellulose polymer chains held together by strong hydrogen bonding and van der Waals forces. The individual cellulose chains are linear condensation polymer molecules made up of anhydroglucose units joined together by β-1,4 glycosidic bonds with degrees of polymerization (DP) ranging, typically, from 1,000 to 15,000 units. The high crystallinity of cellulose, while imparting structural integrity and mechanical strength to the material, renders it recalcitrant towards hydrolysis aimed at producing glucose—the feedstock for producing fuels and chemicals—from this polysaccharide. In general, neither the water molecules nor the catalysts for hydrolysis (saccharification) are able to easily penetrate the crystalline matrix.
Cellulose hydrolysis to glucose is most often catalyzed using mineral acids or enzymes (cellulases). Cellulase hydrolysis is preferred over mineral acid hydrolysis for several reasons: acid hydrolysis leads to formation of undesirable degradation products of glucose that significantly lower glucose yield and inhibit subsequent fermentation; requires expensive corrosion-resistant materials; and poses disposal problems. Glucose degradation products observed with acid pretreatment or hydrolysis include hydroxymethyl furfural (HMF) and furfural which inhibit downstream fermentation to ethanol.
On the other hand, cellulase enzymes are very specific in their action, producing virtually no glucose degradation products. Cellulases (of fungal or bacterial origin) are in fact a mixture of enzymes which act in concert and synergistically. Special materials of construction are not required with cellulase-catalyzed hydrolysis. However, cellulose hydrolysis in aqueous media suffers from slow reaction rates because the substrate (cellulose) is a water-insoluble crystalline biopolymer. Therefore, the enzymes have to accomplish the hydrolytic decomposition via first adsorbing on the cellulose surface, partially stripping the individual polymer chains from the crystal structure, and then cleaving the glycoside bonds in the chain. Adsorption sites of crystalline cellulose are very limited due to the tight packing arrangement of cellulose fibrils which not only excludes the enzymes but also largely excludes water.
Cellulose is very difficult to dissolve due to the extensive network of inter and intra-molecular bonds and interactions between cellulose fibrils. Ionic liquids have recently been shown to be novel solvents for dissolution of cellulose capable of dissolving large amounts of cellulose at mild conditions (Swatloski, R. P. et al., J. Am. Chem. Soc., 2002, 124, 4974-4975; Zhang, H. et al., Macromolecules, 2005, 38, 8272-8277). Ionic liquids (ILs) are salts that typically melt below ˜100° C. With their low volatility, fluidity at ambient temperatures, and unique solvent properties, ILs comprise a class of prospective solvents that are potentially ‘green’ due to their minimal air emissions. Our invention exploits these properties of ionic liquids to enhance the saccharification of cellulose.